Using the G11/GM-8 Polar Finder

In the early 90's when I got seriously back into
amateur astronomy I purchased a Celestron CG-11 system
which consisted of a C-11 tube assembly mounted on
a Losmandy G11. Since that time, Celestron has gone on
its own way and now packages its own G11 clone mount
with the C-11. The information below pertains
only to the old CG-11 system or standalone G11
or GM-8 mounts produced by
Losmandy
(Hollywood General Machining).

One of the inadequately documented items in the
Celestron system package was the optional polar
finder. The first step was to install the
polar finder. If the documentation you got
also was not adequate for installation or you
don't have it because you bought a used mount,
see my Polar Finder
Installation notes.

Once you have the finder installed, you then
have to figure out what all the marks on the
reticle are used for. The marks were cryptic to
me literally for years because they were not
mentioned at all in the documentation supplied.
However, recently this became much clearer to
me when I acquired a
Kenko Sky Memo single-axis tracking
mount which has the same polar finder,
and I was able to decipher enough of the
Japanese instructions to figure out more of the
subtleties of the polar finder. Both
the Kenko tracker and Kenko polar finder (which is
compatible with the Losmandy mounts) are available
from Hutech Corporation
(http://www.sciencecenter.net/hutech/).

Shown below is a diagram of the reticle view
which has been annotated to make things clearer:

Fig. 1. Polar finder reticle.

The first thing to note is that the finder is
meant to do double-duty. There are markings
applicable to the Northern Hemisphere (red
arrows in figure 1), and there are markings
for the Southern Hemisphere (blue arrows). In
some finders, the lines are actually labelled
with "N.H." and "S.H." The important thing is
to ignore the set that isn't relevant to you.
Since I'm located in the Northern Hemisphere,
that's what I'll discuss below first.

Northern Hemisphere Alignment

The second thing to note is that the
constellations shown (Cassiopeia and the Big
Dipper) will not be visible in the finder
itself. These are used in the first step,
which is to rotate the reticle until the
constellation references approximately match
the current sky orientation. This should be
done after first orienting the mount well
enough to at least place Polaris in the
view of the finder.

Once the coarse rotation has been set, you can
then place Polaris in the gap (indicated in
figure 2) using the azimuth and elevation
adjustments of the mount. In some polar finders, the
words "Place Polaris Here") are also etched on
the reticle. Note that as shown in figure 2,
the exact position of Polaris changes, depending
on epoch.

Fig. 2. Northern hemisphere star placement.

Then, using a combination of the azimuth and
altitude adjustment and fine tuning of the
reticle rotation, place the second brightest
star (Delta Ursa Minor) in the
gap indicated. In some finders, you may
find the words "Place Second Star Here."
The three sets of lines/gaps
are for the epochs 1990, 2000, and 2010. In
the diagram above, the red dot in the middle
position indicates the epoch 2000 position.
These may be marked cryptically with "90",
"00", and "10" in some finders as shown in
figure 1.

If you have a dark observing location, you may
also be able to see a faint third star. Place
this in the third (unlabeled) set of
lines/gaps, again using the altitude, azimuth,
and possibly rotation of the reticle. If I am
able to use the third star, I generally find
that I am quite accurately polar aligned, and
this considerably reduces the amount of time
necessary to finish off with drift alignment.
Note that this star is designated 51 Cep in my
Kenko Japanese language documentation. This is
apparently not a commonly used designation (I
checked The Sky version 4, the Millennium Star
Atlas, and a few others). As far as I can
tell, it corresponds to SAO 1168, a magnitude
5.3 star.

Gotcha! #1 -- Wrong Finder Magnification

The procedure above sounds pretty
straightforward, and if you're lucky, that's
all there is to it. However, if you are
unlucky like me, you got one of the defective
polar scopes that came with some CG-11
systems! These have the problem of
incorrect magnification relative to the
reticle. In my setup, the two stars end up
sitting not in the gaps but on the lines closer
to the center (indicated by red spots in the
figure 3). If you don't know this, you'll
undoubtedly have pulled out some hair trying to
figure out why you just can't get the stars to
fit.

Fig. 3. Northern hemisphere star placement in
bad reticles (2000).

The Story Behind the Bad Polar Finders
-- When this topic came up on the Losmandy
Users Group list recently, Scott Losmandy
explained that the polar finders were difficult
to get at the time that the Celestron CG-11 systems were
being produced. So after the first few hundred
units, Celestron went
and had their own finders produced for them and
it is these scopes which were defective --
specifically, they have the wrong magnification
for the reticle markings. These scopes can be
distinguished by the words "Put Polaris Here"
on the reticle. As far as I am able to tell,
new polar scopes sold today are fine.

Gotcha! #2 -- Reticle Illumination Too Bright

Another common problem is that the illuminator
that comes with the polar finder is often too
bright, obscuring the dim second and third
stars. To take care of this problem I now have
a custom illuminator power supply box for my
G-11 (the Kenko mount has an adjustment built
in) which has a rotary selector to brighten or
dim the illuminator as required. An
alternative low tech solution is to take a
black marker or paint or nail polish and paint
the LED down to the point where it isn't
blinding you.

If you don't have the resources to make your
own dimmer and the kludge solutions above don't
appeal to you, a third-party product which
recently came to my attention is the the
AstroElectric Polar Scope Dimmer Controller,
a nicely done low cost unit available on the
web from Mike Cressy at
http://www.4saleusa.net/astroelectric/.
Just unplug your polar scope battery pack and
plug this in. The knob controls the intensity
of the LED, and unplugging turns off the unit
which contains its own pair of AA batteries.
I am told newer units also switch off by turning
the intensity knob all the way down.

Gotcha! #3 -- Mis-centered Reticle

One final item which may trip you up is the
possibility that the reticle may not be
centered properly. The symptom of this problem
is excessive dec tracking error even though you
have carefully aligned your mount with the
finder. To check and possibly adjust this, you
will have to first carefully drift align your
mount, then try to rotate the reticle to its
proper position. If you find that you cannot
do this, you can try adjusting the 3 tiny
setscrews (0.035" hex wrench on most finders)
which are on the same surface that
the illuminator screws into. Some users have
reported cracking the reticle by tightening the
screws too much so extreme care should be used.
Please note that I have not done this myself,
so I can't say how easy it is to do (or mess
up). If you don't feel confident doing this,
it may be best to simply note the offset for
the pole position relative to the markings on
the reticle.

Southern Hemisphere Alignment

Southern Hemisphere alignment appears to be
harder to do because of the lack of bright
stars, but the basic procedure is the same.
In this case, the Kenko mount instructions
recommend first using a compass to set the
mount up in approximate alignment towards the
pole.

Next, rotate the reticle until the Southern
Cross and Alpha Eridani are in the correct
approximate orientation (again not actually
visible in the reticle).

Finally, place the stars of
Octans in the indicated positions using the
mount's altitude and azimuth adjustment as well
as fine tuning of the reticle rotation. Note
that the edges of the tick marks are used also
to indicate the exact positions for the epochs
1990 and 2010.

Fig. 4. Southern hemisphere star placement.

Since both reference stars are practically
in line with the pole, I would expect that
alignment would not generally be as good as for
the Northern Hemisphere. I haven't had a chance
to personally try the Southern Hemisphere
procedure, so someone down there will have to
let me know if it works as advertised.

At any rate, no matter how well you set up
using the polar scope, final adjustment for
astrophotography via a telescope or long
telephoto lenses must be done with drift
alignment. For wide angle photography away
from the poles, this alignment may be adequate.